1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to spin down circuitry having a current limit circuit for enhancing power and braking control.
2. Description of the Prior Art
When the disk spins down in a disk drive it is important to park the head before the air bearing dissipates to prevent damage to the head and/or the disk. In disk drives wherein the head is parked in a landing zone on the disk, it is also important to brake the spindle motor as quickly as possible to minimize head wear. Conventionally, during a power failure the head is parked using the back EMF (BEMF) voltage present over the motor windings due to the angular momentum of the spindle and disk assembly. The current induced by the BEMF charges a capacitor to thereby generate an internal supply voltage which is applied to a voice coil motor (VCM) to park the head. Once the head is parked, a braking torque is applied to the spindle motor to stop it from rotating as quickly as possible in order to minimize head wear.
The spin down performance of a disk drive impacts several design considerations. In lower end drives, for example, cost is of particular concern. If power is generated more efficiently during power failure mode, then the spindle and VCM motors can be less efficient and therefore less expensive. In higher end drives (drives operating at higher RPM), it is important to brake the spindle motor quickly to reduce head wear if the head is parked in a landing zone on the disk. However, it is also important to generate sufficient power to compensate for xe2x80x9chead bouncexe2x80x9d to prevent the head from bouncing away from the parking latch and back onto the user area of the disk. Head bounce can limit the minimum RPM required to successfully park the head, which increases head wear since it takes longer to brake the spindle motor if rotating at a higher RPM once the head is parked. In disk drives that employ ramp loading, it is important to generate sufficient power to park the head on the ramp using an active parking algorithm.
Prior art techniques are known for xe2x80x9cboostingxe2x80x9d the internal supply voltage generated during power failure by periodically shorting the spindle motor windings which increases the available power. For example, U.S. Pat. No. 5,504,402 discloses a boost circuit for boosting the internal supply voltage by periodically grounding the spindle motor windings using a grounding switch. When the grounding switch is turned on (grounded), a current builds in the spindle motor windings due to the inductance and the BEMF. When the current reaches a predetermined level, the switch is turned off so that the current stored in the spindle motor windings charges a capacitor which boosts (and filters) the internal supply voltage. When the internal supply voltage reaches a predetermined level, the grounding switch is turned back on in order to recharge the current in the spindle motor windings.
There are several drawbacks associated with the head parking technique described in the aforementioned ""402 patent. The boost circuit effectively brakes the motor, thereby reducing the time necessary to stop the disk once the head is parked. However, in the ""402 patent, the boost circuit and associated braking action are disabled once the internal supply voltage reaches the predetermined level. Further, using a current controlled feedback loop to regulate the voltage limits the amount of power generated by the boost circuit. In addition, the current controlled loop places constraints on certain system dynamics, such as spindle speed, inductance in the windings, and hysteresis in the comparator, which limits design flexibility.
Another problem with prior art techniques in general is the inability to safely park the head during a power failure unless the disk is rotating fast enough so that sufficient power is available to retract the head. Thus, the head is typically positioned over the landing zone while the disk is still rotating at a high RPM resulting in undesirable head wear. The landing zone is typically textured to reduce the stiction force during spin up; however, this textured surface also wears on the head during spin down. Because prior art techniques position the head over the landing zone while the disk is still spinning at a high RPM, head wear increases due to the increased time to brake the spindle motor. The prior art boost circuit alleviates this problem somewhat by increasing the internal supply voltage, thereby enabling head parking at a lower RPM which reduces the braking time while the head is over the landing zone. However, further improvements are attainable.
There is, therefore, a need to improve upon prior art techniques for generating an internal supply voltage used to park the head in a disk drive during a spin down mode. In particular, there is a need to generate power more efficiently in order to reduce the efficiency and cost of the spindle and VCM motors. In addition, there is a need to continuously brake the spindle motor during the head parking operation in order to stop the disk rotating as quickly as possible, thereby reducing head wear. Further, there is a need to increase the power generated by a boost circuit as well as to improve power management so that head parking is viable at a lower RPM which further reduces head wear. There is also a need to generate sufficient power to park the head on a ramp using an active parking algorithm for drives employing ramp loading. Still further, there is a need for spin down circuitry which is less dependent on system dynamics in order to increase design flexibility and to enable the same spin down circuitry to be employed over a wide variety of disk drives.
The present invention may be regarded as a disk drive comprising a disk, a head actuated radially over the disk, and a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings and a rotor rotatable at a variable spin rate, wherein the rotor generates a back EMF (BEMF) voltage across the windings proportional to the spin rate of the rotor. A plurality of switching elements are coupled to the windings, and switch control logic generates switch control signals applied to the switching elements for commutating the spindle motor during normal operation and for generating an internal supply voltage from the BEMF during a spin down mode, the internal supply voltage for parking the head. A signal generator generates a periodic signal, the periodic signal for periodically grounding the windings during the spin down mode in order to boost the internal supply voltage, wherein grounding the windings applies a braking torque to the spindle motor. A plurality of current detectors is provided, wherein each current detector for measuring a current flowing through a respective one of the switching elements. A current limit circuit generates a control signal which prevents the periodic signal from grounding the windings if the current flowing through any one of the switching elements exceeds a predetermined threshold.
The present invention may also be regarded as a method of enhancing power management in a disk drive during a spin down mode, the disk drive comprising a disk, a head, a voice coil motor (VCM) for actuating the head radially over the disk (the VCM having a voltage control input), and a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings and a rotor rotatable at a variable spin rate wherein the rotor generates a back EMF (BEMF) voltage across the windings proportional to the spin rate of the rotor. A plurality of switching elements are coupled to the windings, and switch control logic generates switch control signals applied to the switching elements for commutating the spindle motor during normal operation and for generating an internal supply voltage Vi from the BEMF during a spin down mode, the internal supply voltage Vi applied to the voltage control input of the VCM for parking the head. A signal generator generates a periodic signal, the periodic signal for periodically grounding the windings during the spin down mode in order to boost the internal supply voltage, wherein grounding the windings applies a braking torque to the spindle motor. A current flowing through each one of the switching elements is measured, and the periodic signal is prevented from grounding the windings if the current flowing through any one of the switching elements exceeds a predetermined threshold.